Aparecida Schunck Flosi Palmeira -- the 67-year-old mother of Ecclestone's Brazilian wife Fabiana Flosi -- had been missing since she was kidnapped on July 22.
Authorities said in a short statement she had been "liberated" and was "unharmed," adding that they had arrested two suspects.
Local Globo TV showed the woman hugging waiting relatives as she arrived at a Sao Paulo police station under police escort.
A demand for a ransom of US$188 million was made to the family via e-mail, but had not been paid, O Estado de Sao Paulo daily reported. Another newspaper, the weekly Veja, reported that the ransom demand was US$36.6 million.
Police did not respond to AFP requests for information.
Kidnappers posing as delivery men abducted her from her home in the Jardin Santa Helena area of southern Sao Paulo, neighbours told AFP last week.
She is the mother of Ecclestone's 38-year-old Brazilian wife Fabiana Flosi, who met the 85-year-old billionaire while working for the Interlagos Grand Prix in 2009.
The couple married in Switzerland and now live in London.
Kidnappings were common in Brazil in the late 1990s and the early 2000s, but dropped with more intensive policing and the creation of a specialized anti-kidnapping department.
Relatives of footballers were particularly targeted, including the mother of Brazilian international star Robinho in 2004. She was held for 41 days in the outskirts of Sao Paulo.
